Method for controlling a belt-type continuously variable transmission and a friction clutch in a vehicular drive line

ABSTRACT

The invention provides for a vehicle drive line provided with an engine ( 1 ) capable of generating an engine torque (Te), a continuously variable transmission ( 2 ), a driven wheel ( 5 ) and two friction clutches ( 3, 33 ), a first clutch ( 3 ) being positioned in the drive line between the engine ( 1 ) and the transmission ( 2 ) and a second clutch ( 33 ) being positioned between the transmission ( 2 ) and the driven wheel ( 5 ), wherein a torque (Tc-max) that is transmissible by the first clutch ( 3 ) and a torque (Tc-max) that is transmissible by the second clutch ( 33 ) are both less than a torque (Tt-max) transmissible by the transmission ( 2 ) and, at the same time, are both essentially equal to or slightly higher than the engine torque (Te).

The present invention relates to a method for controlling a belt-typecontinuously variable transmission and a friction clutch in a vehiculardrive line. Such a control method is well known and, for instance, isdescribed in U.S. Pat. No. 4,606,446 in relation with a transmissionhaving two variable pulleys and a belt wrapped around and in frictionalcontact therewith. The invention also relates to a method for designingsuch a transmission.

A problem generally encountered in belt-type transmission is that slipbetween the friction components thereof, i.e. its belt and pulleycomponents, needs to be avoided or at least may occur only to a limitedextend, since excessive slip results in excessive wear and tear of thesecomponents. To prevent such slip, a sufficient normal force is to beapplied between the pulleys and the belt, which normal force is oftendetermined in dependence on the engine torque to be transmitted by thetransmission increased by a safety margin. The safety margin isintroduced because the engine torque is not always accurately known and,moreover, because sudden torque level variations, i.e. torque jolts, mayalso be introduced into the drive line both by the engine itself and viathe driven wheel or wheels, e.g. when driving over a pothole in theroad.

It has been a long-standing development aim to minimise the said safetymargin, since the efficiency and durability of the transmission may besignificantly improved thereby. A solution has been found in controllingthe friction clutch in such a manner that the torque that is transmittedor transmissible thereby, which is denoted the clutch slip torquehereinafter, is less than the torque that is transmissible by thetransmission, i.e. between the belt and each pulley thereof. This lattertorque level is denoted the transmission slip torque hereinafter.According to U.S. Pat. No. 4,606,446 the clutch may thereby bepositioned either upstream or downstream of the transmission withrespect to the flow of power from the engine to the driven wheels of thevehicle.

Whenever a torque jolt occurs in such system, the clutch slip torque isexceeded before the transmission slip torque, whereby a slipping of theclutch increases and the mechanical energy represented by the torquejolt is at least partly dissipated into heat. Actually, the clutch sliptorque will increase somewhat during the torque jolt because of theincreased slipping of the clutch, but such increase is comparativelysmall and can normally be accommodated by the transmission that iscontrolled to have a slip torque that is higher than the initial clutchslip torque.

However, a problem does occur with the above system and control method,because in dependence on the upstream or downstream position of theclutch relative to the transmission, either the torque jolts introducedby the engine or by the driven wheel will first travel through thetransmission before reaching the clutch. Then, the torque jolt will thenat least be partly dissipated in the transmission, which is undesirableper se. Such problem could be obviated by again applying a comparativelyhigh safety margin, which would of course completely nullify theoriginal development aim.

In practice only a downstream position of the clutch is applied incombination with the known control method, because the torque jolts thatare introduced into the drive line via the driven wheels are supposed tobe less predictable and more severe than those originating from theengine. Still, from a transmission design, i.e. hardware layout, pointof view an upstream clutch can be more easily and efficiently integratedin the transmission. Also, during operation, the clutch that is arrangedupstream relative to the transmission is mechanically loaded far lesswhen compared to the downstream clutch in terms of both the maximumtorque and the maximum rotational speed to be transmitted thereby.Moreover, it is Applicant's experience that also the torque joltsoriginating from the engine, especially in case of a Diesel engine, canbe of a magnitude that is detrimental to the transmission's efficiency,i.e. that require a considerable safety margin to be applied to thenormal force, in particular at a relatively low nominal engine torquelevel.

The present invention aims to improve on the known design, in particularby improving the protection of the transmission against torque joltswhether originating from the engine or from the road. According to theinvention this aim is realised by the drive line of claim 1. More inparticular, the invention aims to also reconcile the above-mentionedengineering preference to position the clutch upstream, i.e. between theengine and the transmission, with such protection of the transmissionagainst torque jolts. Such aim may according to the present inventioneven be realised in a drive line of known design by applying the controlmethod of any one of the independent claims 2, 4 and 6.

In the drive line of claim 1 two clutches are provided, a first clutchis positioned upstream of the transmission and a second clutch ispositioned downstream of the transmission. The clutches are arrangedsuch in the drive line that the respective clutch slip torque levelsthereof are both less than transmission slip torque and, at the sametime, are essentially equal to or possibly somewhat higher the enginetorque level. In such a drive line the transmission is effectivelyprotected against all torque jolts, irrespective of whether theyoriginate from the engine or the driven wheel. Consequently, the saidsafety margin can be favourably small. It is remarked that some existingdrive line designs already include two clutches, such as the lock-up orbridging clutch of a torque converter or a drive-off clutch and theclutch of a planetary or DNR-gearing, which accordingly merely have tobe arranged and possibly made suitable for clutch slip torque control.

However, according to the invention, even when the drive line isprovided with only a single clutch suitable and arranged for clutch sliptorque control, it is possible to improve the protection of thetransmission against torque jolts by applying the control method ofclaim 2.

It is understood that when such clutch is arranged on the “other” sideof the transmission in relation to the origin of a torque jolt, aslipping of the belt relative to a pulley in the direction of rotationthereof will unavoidably occur due to a torque jolt that exceeds thesaid safety margin. Still, Applicant has observed that the belt iscapable of accommodating a considerable amount of slip relative to thetransmission pulleys without being damaged at all. Experiments haveshown that during transmission operation such capacity to accommodatebelt slip up to a critical slip value varies at least with the level ofthe said normal force, the rotational speed of the belt and with thegeometric transmission ratio. For each transmission ratio and beltrotational speed such relationship between the critical slip value andthe normal force level is described by—and is hereinafter referred toas—a critical damage curve. It was found that in terms of the smallestallowable slip value at a given level of the normal force, the mostdecelerating transmission ratio, the so-called Low-ratio, in combinationwith a small belt rotational speed represents the most criticalsituation. In contrast, in the most accelerating transmission ratio, theso-called Overdrive-ratio, in combination with a large belt rotationalspeed a large amount of belt slip can be accommodated by thetransmission.

In a so-called slip characteristic map of the transmission the criticaldamage curves of several transmission ratios are included, whereby itmay be interpolated on or between such predetermined critical damagecurves to obtain the current critical slip value, i.e. that is valid forthe prevailing operational conditions of the transmission. The existenceof this slip characteristic map proves that the transmission may besafely subjected to a torque jolt, as well as to the belt slip that iscaused thereby, up to the extent measured in the above experiment. Bythe application of the clutch control in accordance with the presentinvention the transmission efficiency is optimised while the belt slipincrease in response to the torque jolt is limited sufficiently for notexceeding the critical slip value, as will be explained in more detailin the following, with reference to the figures, wherein:

FIG. 1 is a schematic representation of a vehicular drive line providedwith an engine, a friction clutch, a continuously variable transmission,a final drive mechanism and driven wheels, as well as a control means;

FIG. 2 is a graph illustrating the transmission's capacity toaccommodate a speed difference between its belt and pulley components inrelation to the level of the normal force applied there between and thegeometric transmission ratio;

FIG. 3 is a diagrammatic functional representation of the drive line ofFIG. 1 illustrating the effect of the occurrence of a torque jolt;

FIG. 1 depicts a vehicular drive line, which subsequently, i.e.downstream in the direction of the driving force, comprises an engine 1,a friction clutch 3, a belt-type continuously variable transmission 2, afinal drive 4 mechanism including a differential gearing and two drivenwheels 5. Drive shafts 13, 32, 24 and 45 are provided between anddrivingly connect each of the above drive line components 1-5. Thetransmission 2 comprises two, schematically indicated pulleys 21 and 22and a drive belt 23 wrapped around and in frictional contact with thepulleys 21, 22. Each pulley 21, 22 comprises two conical pulley discsthat during operation are pushed towards each other, such that a normalforce Fn is effected between the discs of each pulley 21 and 22 and thebelt 23.

Further, control means are schematically indicated in FIG. 1, whichmeans comprise an electronic control unit or “ECU” 6, three rotationalspeed sensors 7, 8 and 9 for measuring a clutch input side (i.e. shaft13) rotational speed Ci, a clutch output side (i.e. shaft 32) rotationalspeed Co and a transmission output side (i.e. shaft 24) rotational speedCv and two actuators 9 and 10 for a controlled activation of the clutch3 and the transmission 2 respectively, e.g. by adjusting a clutchengagement force Fc as well as the pulley normal forces Fn-21 and Fn-22in response to one or more operating parameters of the drive line.

Hereby, it is known to arrange and operate the ECU 6 such that a torquethat is transmitted or transmissible by the clutch 3 is maintained at alower level than the torque that is transmissible by the transmission 2,i.e. the transmission slip torque. In this arrangement, when a torquejolt is introduced in the drive line via a driven wheel 5, the clutch 3will not merely transmit such increased torque level, but rather will toa large extend dampen the torque jolt by dissipating the mechanicalenergy associated therewith by an increase (or decrease) of the clutchslip Cs, defined as the quotient Ci/Co between the clutch input sidespeed Ci and the clutch output side speed Co. Accordingly, the driveline and in particular the transmission 2 is protected against a severeslipping of its belt and pulley components 23, 21 and 22. However, sincethe clutch 3 is arranged upstream from the transmission 2, the latterwill experience an additional torque anyway. This additional torqueoccurs in response to the torque jolt, because the belt 23, thetransmission input pulley 21, the shaft 32 connecting the clutch 3 tothe transmission 2 and any output side components of the clutch (clutchplates, clutch fluid etc.) have to be accelerated (or decelerated) to beable to influence (i.e. increase or decrease) the clutch slip Cs. Thisadditional torque will, however, not result in critical damage, providedthe belt slip Bs caused thereby is limited to the extent that a criticalslip value CBs is not exceeded thereby.

According to the invention such limitation of the belt slip Bs isrealised by providing a further clutch 33 in the drive line that isarranged downstream from the transmission 2 and that is likewisearranged and controlled such that a torque that is transmitted ortransmissible thereby is maintained at a lower level than thetransmission slip torque. Alternatively, such limitation can accordingto the invention also be realised by controlling the transmission 2 andclutch 3 in accordance with the control method according to the presentinvention that is described in the following.

First of all, a typical property of the belt-type transmission will beexplained with reference to the FIG. 2. It is known that this type oftransmission 2 is at least to some extent able to withstand slippage,i.e. relative movement, between either pulley 21, 22 and the belt 23,which slippage is denoted the belt slip Bs hereinafter. FIG. 2illustrates this feature for a specific type of belt 23 based onexperimental data. In this figure it is defined as a function of thesaid normal force Fn and the degree of belt slip Bs, whether or notunder the defined conditions the transmission 2 would be criticallydamaged, i.e. damaged to the extent that its normal or nominal operationis adversely affected. Hereby the so-called critical damage curves A, Band C in FIG. 2 indicate the maximum permissible or critical value CBsfor the belt slip Bs, beneath which, indicated by arrows I, no suchcritical damage occurs and above which, indicated by arrows II, suchdamage does indeed occur. Accordingly, the transmission 2 is at least tosome extent able to withstand belt slip Bs.

FIG. 2 also shows that the critical belt slip CBs is much smaller in themost decelerating or Low-ratio of the transmission, which is representedby curve A, than in its most accelerating or Overdrive-ratio representedby curve C, i.e. for the same degree of belt slip Bs damage occurs tothe drive belt 23 or a pulley 21, 22 already at a lower level of thenormal force Fn in the Low-ratio compared, for example, theOverdrive-ratio. Further, the critical damage curve B that alsorepresents the Low-ratio was measured at an elevated rotational speed ofthe belt in comparison with the curve A, which show that the criticalbelt slip CBs increases as the belt rotational speed increases. Finally,it can be seen that the critical belt slip CBs generally decreasessignificantly with an increasing level of the normal force Fn. Thus, asa result of the above experiments, it was found that when a torque jolttravels through the transmission 2, it should be safely transmittedthereby provided that the actual belt slip Bs caused by the increasedtorque level does not exceed the relevant, i.e. current critical beltslip value CBs.

FIG. 3 illustrates the physics of a—in this example—positive torque joltTj introduced into a highly simplified drive line via the load, i.c. thetransmission output side shaft 24 that is rigidly connected to thedriven wheels 5 thereof. For the sake of simplicity, the drive line ofFIG. 3 does not include any gearing-ratios and/or auxiliary drives.Also, any mechanical losses are not taken into account.

On the left-hand side of FIG. 3, the normal, steady state operation ofthe drive line is illustrated, wherein the engine generates an enginetorque Te that is transmitted via the friction clutch 3 and the frictiontransmission 2 to the transmission output side shaft 24. In such steadystate, a sufficient engagement force Fc resp. Fn is applied to theclutch 3 and the transmission 2 respectively, such that all of theengine torque Te, the torque Tc transmitted by the clutch 3, the torqueTt transmitted by the transmission 2 and the torque Tl taken up by theload 24 are the same.

Now, in the right-hand side of FIG. 3 the said torque jolt Tj isintroduced into the drive line and works on the transmission output side24. If by such torque jolt Tj a maximum torque transmissible by thetransmission Tt-max, which can be defined in terms of the engine torqueTe and a transmission safety margin, i.c. expressed in terms of a safetyfactor Sft according to:Tt-max=(1+Sft)*Teis exceeded, i.e. if:Tj+Te>Tt-max=>Tj>Sft*Tethan the transmission output side shaft 24 will accelerate faster thanthe transmission input side shaft, i.e. the clutch output side shaft 32.by this acceleration of the transmission output side shaft 24 relativeto the clutch output side shaft 32 a slipping of the transmissionfriction components, the said belt slip Bs, will increase. Thus suchbelt slip increase ΔBs is determined by how closely the acceleration α₃₂of the clutch output side shaft 32 can follow the instantaneousacceleration α₂₄ of the load 24 during the duration t of the torque joltTj, which factor is determined by the combined inertia Ip of the driveline components to be accelerated and the amount of torque Ti availablefor such acceleration according to:

${\Delta\;{Bs}} = {\int_{0}^{t}{\left\lbrack {{\alpha_{24}(t)} - \frac{Ti}{Ip}} \right\rbrack\  \cdot {\mathbb{d}t}}}$wherein:Ti=(Sft−Sfc)*Te with Sfc<Sftwith Sfc being a clutch safety factor defined according to:Tc-max=(1+Sfc)*Tewherein Tc-max is the maximum torque transmissible by the clutch 3.

Hereby, it is remarked that in the simplified drive line of FIG. 3 thecombined inertia Ip is in fact determined only by the clutch output sideshaft 32. However, in a practical drive line design the combined inertiaIp is not only determined by the clutch output side shaft 32, but alsoby the transmission input pulley 21 and all output side components ofthe clutch (clutch plates, clutch fluid etc.), i.e. at least when theslipping of the belt 23 occurs relative to the transmission input pulley21. In situations where the slipping of the belt 23 occurs relative tothe transmission output pulley 22, the said combined inertia Ip furtherincludes the inertia of the belt 23, as well as a factor representingand corresponding to the transmission ratio defined as the quotientCo/Cv between the clutch output side speed Co (i.e. the transmissioninput side speed) and the transmission output side speed Cv. Hereby, itis remarked that it is usually appropriate to assume that the saidslipping of the belt 23 occurs relative to the transmission outputpulley 22.

Thus, with the above system and control method the belt slip increaseΔBs in response to the torque jolt Tj is favourably reduced, which inturn results either in the belt slip Bs not exceeding the critical beltslip value CBs at all, or at least in the belt slip Bs taking more timeto reach such critical belt slip value CBs. This additional time may befavourably used to invoke countermeasures in response to the occurrenceof the torque jolt Tj for preventing or limiting the increase of thebelt slip Bs, such as:

-   -   increasing the transmission safety factor Sft by actively        reducing the engine torque Te, or by increasing the level of the        normal force Fn, or by applying both such measures        simultaneously, or    -   actively changing the transmission ratio Co/Cv to reduce the        said torque jolt Tj at the transmission output pulley 22.

Hereby, the occurrence of the torque jolt Tj may itself be correlated toand/or detected by the said acceleration α₂₄ of the load 24.

According to the invention it may be concluded from the above analysisthat the efficiency of the transmission may be improved by reducing thenormal force Fn that is applied in response to an engine torque level Tein a reliable manner, i.e. whilst avoiding the said critical damage dueto a severe slipping of the belt 23 relative to a transmission pulley21, 22 from occurring, by means of one or more of the followingmeasures:

-   -   designing the drive line such that the said combined inertia Ip        is comparatively small;    -   applying a small clutch safety factor Sfc, preferably a factor        of zero;    -   adapting the said transmission safety factor Sft during        operation based on the said slip characteristic map, preferably        by increasing and/or decreasing it at least in relation to, but        preferably in proportion with one or more of:        -   the transmission ratio Co/Cv,        -   the inverse value of the belt rotational speed, or        -   the inverse value of the current critical belt slip value            CBs, e.g. as provided by the slip characteristic map of the            transmission;    -   calculating the said normal force Fn that is required for the        maximum transmissible torque of the transmission Tt-max to be        equal to the engine torque Te increased by a safety margin that        is large enough to ensure that the increase in belt slip ABs in        response to the largest occurring torque jolt Tj does not exceed        the smallest occurring critical belt slip value CBs.

Further on this latter measure of determining a suitable, i.e.sufficient transmission safety margin based on the slip characteristicmap of the transmission, it is remarked that a fixed, i.e. safety marginof between 2 to 7 kN, preferably around 4 kN added to the normal forceFn-min that is minimally required to transmit the engine torque Te (i.e.Fn-min@Tt-max=Te) was found to be generally applicable, which means thatthe above defined transmission safety factor Sft actually increases asthe engine torque Te decreases, i.e. is inversely proportionaltherewith. Favourably, this latter feature takes into account that theengine torque Te and the normal force Fn determined by the ECU 6 areusually subject to an absolute error. Thus at a relatively low enginetorque Te the normal force Fn applied in response thereto is subject toa relatively large uncertainty, which requires a relatively largetransmission safety factor Sft. When such uncertainty is comparativelysmall, a more or less constant transmission safety factor Sft cansuffice.

An additional advantage of such control strategies including atransmission safety factor Sft, which either is constant or increases inrelation to the engine torque Te, is that the said amount of torque Tithat is available for acceleration of the clutch output side shaft 32,which torque Ti is proportional to the transmission safety factor Sftminus 1 times the engine torque Te, can be maintained at an acceptablelevel even at the said relatively low engine torque Te.

Thus, according to the invention, when applying one or more of the abovemeasures, the transmission 2 is favourably and effectively protectedagainst critical damage caused by torque jolts Tj travelling upstreamthrough the drive line even though the clutch 3 is placed upstream fromthe transmission 2.

Finally, it is remarked that the above analysis, although focussing onthe positive torque jolt Tj that is introduced into the drive line viathe load 5 with the clutch 3 being arranged upstream from thetransmission 2, in principle (mutatis mutandis) is also valid for anegative torque jolt or for any torque jolt originating from the enginein case the clutch 3 is arranged downstream from the transmission 2 inthe drive line.

The invention claimed is:
 1. Method for controlling a friction clutch(3) and a continuously variable transmission (2) provided with a drivebelt (23) wrapped around and in frictional contact with two variablepulleys (21, 22) in a vehicular drive line, which method includes thestep of activating the clutch (3) and the transmission (2) in relationto an engine torque (Te) generated by an engine (1) of the drive line,at least effecting that a torque level (Tc-max) transmissible by theclutch (3) is smaller than a torque level (Tt-max) transmissible by thetransmission (2) and that a safety margin is realised between the torquelevel (Tt-max) transmissible by the transmission (2) and the enginetorque (Te), wherein, said safety margin is adapted during operation byincreasing or decreasing said safety margin at least in relation to andin proportion with one or more of: the value of a transmission ratio(Co/Cv) of the transmission (2); the inverse value of a rotational speedof the belt (23); and the inverse value of a current critical belt slipvalue (CBs) at which damage would occur to the transmission (2) to theextent that the transmission's operation is adversely affected thereby,and a normal force (Fn) is applied in the frictional contact between thebelt (23) and the pulleys (21, 22), which normal force (Fn) isdetermined by adding a margin to a nominal normal force (Fn-min) thatwould be required for the transmissible torque (Tt-max) of thetransmission (2) to be equal to the engine torque (Te), which margin ismaintained with the range of 1 to 7 kN during operation of thetransmission (2).
 2. The method of claim 1, wherein the normal force(Fn) is applied in the frictional contact between the belt (23) and thepulleys (21, 22), which normal force (Fn) is determined by adding saidmargin to the nominal normal force (Fn-min) that would be required forthe transmissible torque (Tt-max) of the transmission (2) to be equal tothe engine torque (Te), said margin is maintained with the range of 1 toabout 4 kN during operation of the transmission (2).
 3. Method forcontrolling a friction clutch (3) and a continuously variabletransmission (2) provided with a drive belt (23) wrapped around and infrictional contact with two variable pulleys (21, 22) in a vehiculardrive line, which method includes the step of activating the clutch (3)and the transmission (2) in relation to an engine torque (Te) generatedby an engine (1) of the drive line, at least effecting that a torquelevel (Tc-max) transmissible by the clutch (3) is smaller than a torquelevel (Tt-max) transmissible by the transmission (2) and that a safetymargin is realised between the torque level (Tt-max) transmissible bythe transmission (2) and the engine torque (Te), wherein, said safetymargin is adapted during operation by increasing or decreasing saidsafety margin at least in relation to and in proportion with the inversevalue of a current critical belt slip value (CBs) at which damage wouldoccur to the transmission (2) to the extent that the transmission'soperation is adversely affected thereby, and in response to theoccurrence of a torque jolt (Tj) introduced in the drive line duringoperation one or more of the following measures is invoked: increasingthe safety margin by either one of actively reducing the engine torque(Te) and increasing a normal force (Fn) that is applied in thefrictional contact between the belt (23) and the pulleys (21, 22), orboth; actively changing the transmission ratio (Co/Cv) of thetransmission (2) to counteract the torque jolt (Tj) experienced at antransmission output pulley (22) of the pulleys (21, 22).